With the Blu-ray Disc™ standards using blue-violet semiconductor lasers, it has currently become possible, even for consumer use, to commercialize optical disks having a recording density on the order to 50 GB. In future, a large capacity equivalent to a HDD (Hard Disk Drive) capacity of 100 GB to 1 TB is desired even for optical disks.
However, to achieve such extra-high density in optical disk, a densification technology by a new technique different from a densification technology by shortening a wavelength and increasing NA of an objective lens.
While next-generation storage technologies are being studied, hologram recording technology for recording digital information by using holography has attracted attention.
Hologram recording technology is a technology of recording information on a recording medium by superposing signal light having information regarding page data two-dimensionally-modulated by a spatial light modulator (SLM) on reference light inside a recording medium and causing refractive index modulation in the recording medium by an interference fringe pattern occurring at that time.
At the time of reproducing information, reference light used at the time of recording is applied onto the recording medium, thereby causing a hologram recorded on the recording medium to act as a diffraction grating to cause diffracted light. This diffracted light is reproduced as the same light including the recorded signal light and phase information.
The reproduced signal light is two-dimensionally detected at high speed by using an image pickup element such as a CMOS or CCD. In this manner, the hologram recording technology allows two-dimensional information to be recorded on a recording medium by one hologram at a dash and furthermore allows this information to be reproduced. And, since a plurality of pieces of page data can be overwritten at a location on a recording medium, high-capacity and high-speed information recording and reproduction can be achieved.
An example of the hologram recording technology is disclosed in Japanese Patent Application Laid-Open No. 2004-272268 (PTL 1). This gazette describes as follows: “a multiplexing method and apparatus are disclosed in which holograms are spatially multiplexed by partial and spatial overlapping of the holograms between adjacent stacks. Each stack can further take full advantage of another multiplexing technology, for example, angle, wavelength, phase coding, peristrophic, or fractal multiplexing. The amount equal to the beam waist of signal light for writing a hologram separates the respective stacks of the holograms. At the time of reproduction, a hologram and a hologram adjacent to that hologram are all simultaneously read. With a filter arranged to the beam waist of the reproduced data, the read adjacent hologram is not transmitted to a camera surface. Alternatively, this unwanted reproduction can be filtered by an angular filter on an intermediate surface in an optical system with a limited angle passband”.
Also, an example of a background technology in the present technical field is disclosed in PTL 2. This document describes, as a problem, “an object is to provide a medium for hologram memory in which a light spot for recording data can be accurately applied onto a recording medium” and, as a solution, “a medium for hologram memory is provided in which the medium for hologram memory is configured of a data recording area and a servo information area to allow a light spot for recording data to be accurately applied onto a recording medium”.